Development of an Equivalent Homogenous Fluid Model for Pseudo-Two-Phase (Air+Water) Flow through Fractured Rock

Fracture flow of two-phase mixtures is particularly applicable to the coal mining and coal bed methane projects in Australia. A one-dimensional steady-state pseudo-two-phase flow model is proposed for fractured rock. The model considers free flow of a compressible mixture of air and water in an inclined planar fracture and is based upon the conservation of momentum and the “cubic” law. The flow model is coupled to changes in the stress environment through the fracture normal stiffness, which is related to changes in fracture aperture. The model represents the individual air and water phases as a single equivalent homogenous fluid. Laboratory testing was performed using the two-phase high-pressure triaxial apparatus on 54 mm diameter ~approximately 2:1 height: diameter! borehole cores intersected by induced near-axial fractures. The samples were of Triassic arenaceous fine-medium grained sandstone ~known as the Eckersley Formation! that is found locally in the Southern Coalfield of New South Wales. The sample fracture roughness was assessed using a technique based upon Fourier series analysis to objectively attribute a joint roughness coefficient. The proposed two-phase flow model was verified using the recorded laboratory data obtained over a range of triaxial confining pressures ~i.e., fracture normal stresses!.